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  1. #1
    Join Date
    Jun 2010
    Location
    Northern Virginia
    Posts
    50

    Humidity slowly goes down, then shoots right back up

    My tech replaced my previously bad Ruud RCFL coil today. It's very hot outside (92F) and when my system came back on, it ran for several hours uninterrupted. I wanted to try her out so I set my tstat to 70.

    I watched over several hours as humidity in my house finally started to drop from 65%... to 61%... broke 60.... and eventually got to 55%, by which point it was actually cold in my house, so I rose the Tstat to 72.

    No sooner did the AC kick off than humidity started to rise. In less than an hour, it was back to 65%.

    Mind you, my house has been without proper AC for two months thanks to run-arounds with this tech. I know it's probably soaked and has a lot to pull out. But is that behavior normal? Will it really take 3-4 days before it gets to <50 consistently? I have brand new windows and a pretty well insulated and sealed house.

  2. #2
    Join Date
    Jul 2010
    Location
    DFW -> Texas
    Posts
    446
    Quote Originally Posted by msnerd1980 View Post
    My tech replaced my previously bad Ruud RCFL coil today. It's very hot outside (92F) and when my system came back on, it ran for several hours uninterrupted. I wanted to try her out so I set my tstat to 70.

    I watched over several hours as humidity in my house finally started to drop from 65%... to 61%... broke 60.... and eventually got to 55%, by which point it was actually cold in my house, so I rose the Tstat to 72.

    No sooner did the AC kick off than humidity started to rise. In less than an hour, it was back to 65%.

    Mind you, my house has been without proper AC for two months thanks to run-arounds with this tech. I know it's probably soaked and has a lot to pull out. But is that behavior normal? Will it really take 3-4 days before it gets to <50 consistently? I have brand new windows and a pretty well insulated and sealed house.
    Well when the unit shut off because you set it to low it stayed off so long that the moisture content of the house increased which is what you would expect.

    If the house is more than 10 years old im sure its very drafty which would account for the fast return of moisture.

  3. #3
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,728
    Wow, brought the house down pretty quickly. Kind of surprised it stayed ahead of the humidity at all.

    How do you feel about seeing how cold you can get it? Be an interesting way to back into how the system matches with the house.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  4. #4
    Join Date
    Jun 2010
    Location
    Northern Virginia
    Posts
    50
    That's kind of what I was trying to do. I got home from work at 5pm and it had been running, but was off, because the tech left it at 72 and it was 72 inside. When I set it to 70, it got down to 70 pretty quickly... but couldn't get much beyond that even with it running flat out till about 8pm. At 8:30pm, I set it back to 72. Humidity was 55% at that point, but quickly shot back up to 65%. Now the system is cycling to keep at 72, and it's sitting around 63% RH.

  5. #5
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,985
    Yes, it could take several days & long runtimes with a cold coil to sufficiently reduce the moisture from everything in the home.

    I hope the unit is sized right, & you might want to use 350-CFM per/ton of cooling for hopefully a colder coil & longer run times...

    Set the room t-stat on auto to shut down the indoor blower when the outdoor condenser shuts down.
    Last edited by udarrell; 08-10-2010 at 07:38 PM. Reason: Blower off when condenser off... hope unit sized right

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,432
    Quote Originally Posted by msnerd1980 View Post
    My tech replaced my previously bad Ruud RCFL coil today. It's very hot outside (92F) and when my system came back on, it ran for several hours uninterrupted. I wanted to try her out so I set my tstat to 70.

    I watched over several hours as humidity in my house finally started to drop from 65%... to 61%... broke 60.... and eventually got to 55%, by which point it was actually cold in my house, so I rose the Tstat to 72.

    No sooner did the AC kick off than humidity started to rise. In less than an hour, it was back to 65%.

    Mind you, my house has been without proper AC for two months thanks to run-arounds with this tech. I know it's probably soaked and has a lot to pull out. But is that behavior normal? Will it really take 3-4 days before it gets to <50 consistently? I have brand new windows and a pretty well insulated and sealed house.
    Is your fan on "auto" or "on" mode? There is several pounds of moisture left on your a/c cooling coil. The moisture evaporates back to the home in 30 minutes with the fan on. In the auto mode, it takes upto a couple hours for the moisture to evaporate back to the home. Also occupants put moisture into the home, roughly 1/4 lb. per hour/person.
    For a home to be healthy, you need 75 cfm of fresh air infiltration to purge indoor pollutants. this is an additional 2-3 lbs. of moisture per hour. All things considered, you have a 2-4 lbs. moisture load. A properly set up a/c operating 3/4 of the time with 25^F temp drop and fan in the auto mode will remove the moisture. When the a/c is not operating enough, suggest a good dehumidifier to supplement moisture removal.
    On the issue of moisture in materials without cooling, hot homes have low %RH with high temperatures. The moisture in materials in the home is determined by %RH not the dew point of the air. The materials in a home with 70^F, 65% have a higher moisture content than a home 80^F, 50%RH.
    Do not change anything on your a/c until things settle. 75^F, 50%RH is a good compromise. With extended a/c operation, your a/c should be able to maintain <50%RH. You will need supplemental dehumidification as the cooling load declines to maintain <50%RH.
    Keep us posted.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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